WASHINGTON – Sen. Schumer’s stunning endorsement of Hillary Clinton — three years in advance of the presidential election and before she has even said she’s running — is encountering brush-back from fellow Democrats.

“It was quite surprising,” said Iowa Democratic Sen. Tom Harkin, who spoke to Schumer before his Saturday night speech at a big political dinner in Des Moines — without getting a head’s up.

Schumer urged Clinton to “Run Hillary, run!” and said “when she does,” he’d be right there by her side.

“It’s too early to get into 2016. We’re not even a year into the president’s second term,” Harkin said Tuesday.

Schumer’s PR strategy created maximum impact, although some Democrats are wary of handing the nomination to Clinton without giving other potential candidates a chance.

“The Hillary force is at work. You can feel them out there laying the groundwork despite the denials,” said one longtime Democratic operative.

“By all accounts [Schumer] doesn’t do anything haphazardly. If he’s going to go out on a limb like that I assume somebody somewhere knew about it.”

Referring to a competitive Schumer/Clinton rivalry dating to their time in the Senate, the operative added: “For all the times [Clinton] got Schumed in the past, he wanted to get out front and lay down a marker early to be of assistance.”

Schumer’s tactic undermined the efforts of Vice President Joe Biden, who has made several gestures in the direction of running, but who doesn’t have much support right now among leading Democrats.

“It’s only three years away!” said a sarcastic Delaware Sen. Tom Carper, who hails from Biden’s home state.

“Lets concentrate a little less on who we’re going to be having vying for the nomination of our party, the Republican party, lets figure out how to work together here.”

Carper called any choice by Biden “a decision that’s at least a year away” and referenced a “light-hearted chat” he had with the VP in the Capitol.

“We haven’t talked about it for almost a week,” Carper quipped.

“Sen. Schumer is certainly welcome to champion his home-state colleague. I think it’s awfully early to be looking at the 2016 race,” agreed Sen. Chris Coons (D-Del.). “We have plenty of issues to confront between now and the 2014 election.”

Asked about Biden’s prospects, Coons responded that Biden was “enthusiastically supported in Delaware.”
Asked whether Clinton would have known what Schumer was up to with the endorsement, Sen. John McCain (R-Ariz.) joked, “I’m sure she was shocked – shocked!”

Schumer told The Post Clinton was unaware of his endorsement in advance, but said he was “not commenting on anything in the speech.”

The new book “Double Down” by Mark Halperin and John Heilemann reveals that Obama’s 2012 campaign ran focus groups and poll-tested the effect of dumping Biden from the ticket – a damaging leak that hurts Biden’s public image.